A gerotor pump is a well known type of internal gear pump having a stationary, eccentric ring circumscribing an outer, internal-tooth type gear, which in turn circumscribes an inner, external-tooth type gear mounted on a rotating shaft. The ring and gears are bounded axially by stationary port plates through which the fluid enters and exits. In operation, the meshing of the two gears provides the pumping action. Typically, the ring is made of aluminum and the gears from steel.
Within the aerospace art, as well as other art fields, it is necessary to operate such pumps at low temperatures. For example, aboard aircraft it is sometimes necessary to start and operate the pump inflight where ambient temperatures can get as low as -65.degree. F. When a conventional gerotor pump becomes cold soaked, upon starting it experiences an exceedingly high drag torque. This drag torque remains high until the pump approaches its normal operating temperature, and in extreme cases may prevent the starting or operation of the associated equipment upon which the pump is used.
This undesirable characteristic of conventional gerotor pumps is largely attributable to the conflicting requirements in the design of such pumps. One design requirement is to have sufficiently close operating radial and axial clearances within the pump so that leakage flows are kept to an acceptable minimum during normal operation. Another requirement is that these clearances must be large enough to prevent forceful engagement, (i.e. high drag torque), between the gears and the ring after the gears and ring have undergone thermal contraction during a cold soak.
Thus, there is a need for a gerotor pump that has reduced drag torque at low temperatures in comparison to prior art gerotor pumps.